Fuel cells electrochemically convert fuels and oxidants to electricity and heat and can be categorized according to the type of electrolyte (e.g., solid oxide, molten carbonate, alkaline, phosphoric acid or solid polymer) used to accommodate ion transfer during operation. Moreover, fuel cell assemblies can be employed in many (e.g., automotive, aerospace, industrial, residential) environments, for multiple applications.
A Proton Exchange Membrane (hereinafter “PEM) fuel cell converts the chemical energy of fuels such as hydrogen and oxidants such as air directly into electrical energy. The PEM is a sold polymer electrolyte that permits the passage of protons (i.e., H+ ions) from the “anode” side of the fuel cell to the “cathode” side of the fuel cell while preventing passage there through of reactant fluids (e.g., hydrogen and air gases). The membrane electrode assembly is placed between two electrically conductive plates, each of which has a flow passage to direct the fuel to the anode side and oxidant to the cathode side of the PEM.
Two or more fuel cells can be connected together to increase the overall power output of the assembly. Generally, the cells are connected in series, wherein one side of a plate serves as an anode plate for one cell and the other side of the plate is the cathode plate for the adjacent cell. Such a series of connected multiple fuel cells is referred to as a fuel cell stack. The stack typically includes means for directing the fuel and the oxidant to the anode and cathode flow field channels, respectively. The stack also usually includes a means for directing a coolant fluid to interior channels within the stack to absorb heat generated by the exothermic reaction of hydrogen and oxygen within the fuel cells. The stack also generally includes means for exhausting the excess fuel and oxidant gases, as well as product water.
In some fuel cell systems, the fuel cell is coupled in parallel with an energy storage device (e.g., battery, capacitor, etc.) which is then coupled to a load. Commonly referred to as a hybrid system, peak power from the system is supplied by the energy storage device while the fuel cell provides the average power needs of the application. In most hybrid systems a voltage converter is used to convert the fuel cell stack voltage to the energy storage device voltage. In these types of systems, the fuel cell can operate independently from the energy storage device.
Another type of hybrid system eliminates the need for the voltage converter and couples the fuel cell stack directly to the energy storage device. In this system the fuel cell stack voltage, energy storage device voltage and load voltage are equal. The current output of the fuel cell is therefore dictated by the polarization curve of the fuel cell being used. Therefore, the voltage of the system controls the current output of the fuel cell.
In addition to the energy storage device, many fuel cell systems include a balance of plant that supplies the necessary reactant and cooling fluids for a fuel cell or fuel cell stack. The balance of plant may include devices such as pumps, air compressors, blowers, fans, valves, and sensors. These devices function cohesively to provide power to a load, such as a stationary device or an industrial electric vehicle (e.g., a forklift truck).
An electronic system controller conditions the signals from the sensors and commands the actuators in order to operate the fuel cell system. The software in the system controller is typically designed to optimize one or more aspects of the fuel cell system, such as output power, efficiency, safety, fuel cell life, battery life, etc. In the case of a load such as an industrial electric vehicle, these optimizations can be achieved more easily if the fuel cell system has some knowledge of, or control over, the load.
Thus, there is a need for a means to allow the fuel cell system to communicate with a load, such as an industrial electric vehicle, in order to optimize the operation of the combined fuel cell and vehicle system.